This invention relates generally to driver circuitry for use in high speed logic circuits, and more particularly the invention relates to a driver circuit with improved feedback to facilitate an off transition without ringing.
FIG. 1 is a schematic of a conventional high speed driver circuit for use in high speed logic circuits operating up to 100 MHz and higher. To assist in turning N channel driver transistor 10 off without ringing in response to a positive signal applied to the input of inverter 12, N channel transistors 14, 16 serially provide a feedback from the source of transistor 10 to the gate of transistor 10. Both transistors are momentarily on when the inverter 12 input switches from low to high due to transistor 16 being immediately turned on by the input and transistor 14 being delayed in turning off due to the delay in serial inverters 18, 20 which connect the output of inverter 12 to the gate of transistor 14. This momentary feedback reduces ringing oscillation at the output node Z when transistor 10 is turned off and by maintaining some conduction of the driver transistor during the transition.
Unfortunately, the feedback transistors 14, 16 must be placed in the driver transistor well 11 in the semiconductor chip, thus reducing the area available for the driver transistor or else requiring a larger well size. Further, the driver does not have a global set signal for a high impedance state at the output Z; in order to put the Z bus in a high impedance state, additional logic must be provided in front of the driver. This adversely affects circuit size and speed of operation. Additionally, the driver always draws i.sub.dd current.
The present invention is directed to an improved driver circuit which overcomes these limitations in the prior art driver.